1. Field of the Invention
The present invention relates to a pigment producing interference color. The pigment of the present invention is particularly useful for an automotive finishing paint.
2. Discussion of the Prior Art
A metallic paint containing aluminum flakes or other metal particles is very popular as a finishing paint for automobiles, domestic electric appliances, etc. The aluminum flakes, however, are low in lightness and hard to produce light colors. Therefore, pearl mica has been increasingly used in recent years.
The pearl mica has a structure in which a surface of mica 100 is covered with a titanium dioxide layer 101, as shown in FIG. 8. An incident ray on this pearl mica is divided into a ray A which is reflected on a surface of the titanium dioxide layer 101 and a ray which passes through the titanium dioxide layer 101. The ray passed through the titanium dioxide layer 101 is further divided into a ray B which is reflected on a surface of mica 100, and a ray which passes through mica 100. Reflected rays A and B produce various colors by light interference. For example, when the titanium dioxide layer 101 is 120-135 nm in thickness, pearl mica gives blue by light interference of rays A and B.
A part of the ray passing through the pearl mica reaches a surface of a base coat 102 and is then reflected on the base coat 102. The ray returns back through the pearl mica and exits as a reflected ray C out of the titanium dioxide layer 101 on the incident side. Because the reflected ray C and reflected ray B are mixed and produce white color, interference of the reflected rays A and B is lessened and the interference color is weakened.
Further, the pearl mica is poor in coloring power and hiding power. For example, an automotive metallic coating has been conventionally applied by using a metallic paint and a clear paint in a double-coating-single-curing operation. Thickness of the metallic paint film containing pearl mica at the time of complete hiding is generally 200 .mu.m or more. When the metallic paint containing the pearl mica is applied by the conventional coating operation, color of a base coat is seen through the metallic paint film. Thus, uneven coating occurs here and there and is hard to be corrected.
Therefore, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 215857/1984, it has been conducted to apply a colored intercoat which has the same or somewhat similar color to a given metallic paint containing pearl mica, and then apply the metallic paint containing pearl mica thereon. This method, however, requires as many intercoats as metallic paints and additional coating processes, and has resulted in increased production costs.
In order to overcome the disadvantages of the pearl mica, a coloring mica has been considered. In the coloring mica, as shown in FIG. 9, a titanium dioxide layer 201 formed on pearl mica 200 is covered with an iron oxide layer 202 such as a Fe.sub.2 O.sub.3 layer. This coloring mica gives interference color and inherent color of iron oxide. Accordingly, color power and hiding power of the paint film containing the coloring mica is much enhanced compared with that containing the pearl mica.
The coloring mica pigment offers various colors by changing thickness of the titanium dioxide layer 201 or changing crystal structure of the iron oxide layer 202. However, because the color of this pigment is based on the inherent color of iron oxide, color variety of the pigment is restricted in the range from red to yellow. Further, because the inherent color of iron oxide is turbid, the coloring mica pigment offers less clear color than the inteference color. Research and development have been conducted to add metals such as cobalt, copper, chromium, cadmium to mica pigments for the purpose of increasing the color variety, but have not reduced the paints into practical use because of toxicity and poor durability.
On the other hand, as shown in FIG. 10, Japanese Unexamined Patent Publication (KOKAI) No. 60163/1985 discloses a pigment in which surface of mica 300 is coated with a titanium monoxide layer 301, and then a titanium dioxide layer 302 is formed thereon. Japanese Unexamined Patent Publication (KOKAI) No. 225264/1986 discloses a paint using this pigment. This pigment (hereinafter referred to as black pearl pigment) offers a less reflected ray C because the titanium monoxide layer 301 is dark brown and absorbs many rays passing through. Therefore, interference between reflected rays A and B is hardly disturbed by the ray C and produces strong interference color. The black pearl pigment produces clear color with strong coloring power and hiding power compared with the conventional pearl mica pigment.
The coloring power and hiding power of the black pearl pigment, however, are not strong enough to be used for an automotive finishing paint. Namely, even if the black pearl pigment is contained in the paint by the maximum amount without adversely affecting the paint film properties, the paint film thickness required for complete hiding is as high as 80-100 .mu.m and it is not practical enough.